Why the experts reckon Turner's Modern Rome is so special

This is arguably Turner's finest depiction of an Italian city, and its sun-filled panorama represents the culmination of his fascination with Rome, which lasted for over 20 years. David Moore-Gwyn of Sotheby's says: "This picture has everything – a colourful, relaxed beauty and exquisite detail."

Turner has taken as his viewpoint the top of the Capitoline Hill to depict an unfolding scene.

The spectator's eye is drawn out to the Coliseum. The dreamlike vista combines and presents us with the glories of the past – classical antiquity, the Renaissance, the seat of the Papacy. Turner was interested in the rise and fall of civilisations. When the work was first exhibited, he accompanied it with lines from Byron's Childe Harold: "The moon is up and yet it is not night,/ the sun as yet divides the day with her." But Turner has done more than capture the city in a moment between night and day; he has fused the city's modern life with its historic past.

Contemporary 19th-century life – with its goatherds, processions and people attending to their business – takes place in the foreground.

Mr Moore-Gwyn says the painting "helps to highlight the genius of Turner by revealing the techniques including cross-hatching of dry paint, the working with the bristles of the brush" and:

The thinning out of the paintwork and the subtle nuances of colour. Turner has created a sparkling city bathed in atmospheric light. The painting evokes the very essence and qualities of an Eternal City.